Supply systems of this character are widely used in order to provide standby sources taking over the energization of the load whenever a previously active source drops out for any reason. In order to insure continued operation of other sources in the event of a failure of one source, each source is generally separated from the load by an isolating diode.
Each source conventionally includes a current generator with a so-called rectangular voltage/current characteristic, i.e. with an output voltage which is substantially independent of output current until the latter reaches a maximum value which it maintains while the voltage drops. In the operating range below that maximum value, the generator voltage is controlled by voltage-regulating circuitry including a comparator on the basis of a first input voltage proportional to load voltage and a second input voltage representing a fixed reference level. The first input voltage is obtained via a feedback connection across the load, that connection usually including a resistance network with a voltage divider providing a predetermined step-down ratio.
Even with careful calibration of the several current generators and the associated control units it is practically impossible to make the output voltages of the parallel-connected sources mutually identical. As a rule, therefore, the source with the highest output voltage will dominate inasmuch as the comparators of other voltage regulators will sense a load voltage exceeding the preset level and will therefore throttle the output of the associated current generator. Unless that generator is contributing a significant fraction of the load current, the resulting decrease in load voltage will be promptly compensated by an increase output current of the dominant source, thus causing a further cutback in the output of the remaining source or sources eventually leading to their complete deactivation. This may cause an untimely response of an alarm indicator connected upstream of the isolating diode of the deactivated standby source; more importantly, a subsequent failure of the dominant source will delay the reactivation of the standby source and will cause a momentary drop in load voltage which may be inadmissible in some instances, as where the load is a logic circuit of the TTL (transistor-transistor logic) type. Particularly in the latter instance a maintenance of the load voltage within 5% of its nominal value is essential.
Various attempts have been made to obviate the above drawbacks by insuring that each source supplies a substantial fraction of the total load current, e.g. a minimum of 10 to 20%, under all operating conditions except in the case of its own breakdown. One proposal involves the use of a feedback loop working into a comparison circuit which senses the total load current and controls the contribution of each operative source. This solution, however, is relatively complex even in the simple case of 1:1 redundancy in which the load is fed by only two sources each normally contributing half its current; it is therefore economically justified only with large-scale supply systems.
Another known possibility lies in the use of static feeders in lieu of the astatic current generators with rectangular characteristic referred to above, such feeders having an output voltage which varies inversely with output current. With a pair of static feeders it is necessary, for the purpose of insuring their simultaneous operation, to make their voltage drop equal to at least twice the maximum permissible deviation of the load voltage from its nominal value. This requirement, however, is unacceptable for most low-voltage sources (e.g. of 5 V) used in telecommunication systems.
Still another prior-art solution resides in feeding back the output voltage of each source, taken at a point upstream of its isolating diode, rather than the load voltage available downstream of that diode. Such an arrangement stabilizes the output voltage of each source at a preset value even if that source does not contribute to the load current. The actual load voltage, however, may differ significantly from the stabilized source voltage, on account of current-dependent voltage drops across the diode and in the supply conductors, to an extent which could be as high as 0.5 V. That difference could be acceptable with supply systems operating at high or medium voltages (e.g. of 100 V or 24 V) but not in the low voltage range of about 5 V used in a telecommunication system with tolerance limits of about 2 to 3%.